Antigravity-Review-Fixes: Traversal-Guard, Client-Pooling, Cache-Lock, Doku

Vier verifizierte Befunde aus dem externen Review (Gemini 3.1 Pro):
- app.py: SPA-Fallback gegen Path-Traversal gehaertet (resolve + is_relative_to,
  liefert nur noch Dateien INNERHALB von frontend/dist aus).
- app.py/gateway_proxy.py: geteilter httpx.AsyncClient im lifespan statt neuer
  Client pro /v1-Anfrage (Keep-Alive/Pooling, spart Sockets unter parallelen Agent-Stroemen).
- system.py: check_versions_cached() mit threading.Lock + Double-Check gegen Scan-Stampede.
- AGENTS.md: Zeitzonen-Drift korrigiert (Box laeuft Europe/Berlin, nicht UTC).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/app.py

@@ -11,6 +11,7 @@ import logging
 import os
 from contextlib import asynccontextmanager
 
+import httpx
 from fastapi import FastAPI
 from fastapi.middleware.cors import CORSMiddleware
 from fastapi.responses import FileResponse
@@ -44,11 +45,18 @@ async def lifespan(app: FastAPI):
         tasks.append(asyncio.create_task(memory_svc.auto_dedupe_loop()))
         log.info("Mem0-Auto-Dedupe aktiv (alle %ss, Schwelle %s)",
                  memory_svc.AUTO_DEDUPE_INTERVAL, memory_svc.AUTO_DEDUPE_THRESHOLD)
+    # Geteilter HTTP-Client zur lokalen Engine: Keep-Alive/Connection-Pooling statt neuer Client
+    # pro /v1-Anfrage (spart Sockets/TIME_WAIT unter parallelen Agent-Strömen von Zed/Kilo).
+    app.state.gw_client = httpx.AsyncClient(
+        timeout=httpx.Timeout(connect=10.0, read=None, write=None, pool=10.0),
+        limits=httpx.Limits(max_keepalive_connections=32, max_connections=64),
+    )
     try:
         yield
     finally:
         for task in tasks:
             task.cancel()
+        await app.state.gw_client.aclose()
 
 
 app = FastAPI(title="Mission Control 2.0", version=VERSION, lifespan=lifespan)
@@ -88,13 +96,19 @@ app.include_router(console.router)  # Box-Konsole (ttyd) same-origin durchreiche
 if FRONTEND_DIST.exists():
     app.mount("/assets", StaticFiles(directory=FRONTEND_DIST / "assets"), name="assets")
 
+    _DIST_ROOT = FRONTEND_DIST.resolve()
+
     @app.get("/{full_path:path}")
     def spa(full_path: str):
-        # Falls die Datei direkt in FRONTEND_DIST liegt (z.B. manifest.webmanifest, favicon.ico), liefere sie aus
-        target = FRONTEND_DIST / full_path
-        if target.is_file():
-            return FileResponse(target)
-            
+        # Datei direkt aus FRONTEND_DIST ausliefern (manifest.webmanifest, favicon.ico, …) — aber
+        # NUR innerhalb des dist-Ordners: Pfad auflösen + Traversal (../, absolute Pfade) hart raus.
+        try:
+            target = (FRONTEND_DIST / full_path).resolve()
+            if target.is_relative_to(_DIST_ROOT) and target.is_file():
+                return FileResponse(target)
+        except (ValueError, OSError):
+            pass
+
         index = FRONTEND_DIST / "index.html"
         if index.exists():
             # index.html nie cachen → Browser zieht nach jedem Deploy das aktuelle (gehashte) Bundle.